1. Field of the Invention
The present invention relates to thermal printers and more particularly, to the utilization of inverted acoustic signals for noise cancellation in a thermal printer.
2. Description of Related Art
In the field of bar code symbology, vertical bars of varying thicknesses and spacing are used to convey information, such as an identification of the object to which the bar code is affixed. Bar codes are often printed onto a print media comprising individual paper substrate labels having an adhesive backing layer that enables the labels to be affixed to objects to be identified. Since the bar and space elements have differing light reflective characteristics, the information contained in the bar code can be read by interpreting the reflected light or image pattern from the bar code using known optical scanning systems. In order to accurately read the bar code, it is thus essential that the bar code be printed in a high quality manner, without any streaking, blurring or misregistration of the bar code. At the same time, it is essential that the adhesive backing layer of the labels not be damaged by heat generated during the printing process.
In view of these demanding printing requirements, bar codes are often printed using direct thermal or thermal transfer printing techniques. In direct thermal printing, a print media is impregnated with a thermally sensitive chemical that is reactive upon exposure to heat for a period of time. Thermal transfer printing requires an ink ribbon that is selectively heated to transfer ink to the print media. These two printing techniques are referred to collectively herein as thermal printing.
In operation, a print media is drawn between a platen and a thermal print head of the thermal printer. The thermal print-head has linearly disposed printing elements that extend across a width dimension of the print media. The printing elements are individually activated in accordance with instructions from a printer controller. As each printing element is activated, the thermally active chemical of the ribbon (or print media in direct thermal printing) activates at the location of the particular printing element to transfer ink to the printed area of the print media. The print media is continuously drawn through the region between the platen and the thermal print head, and in so doing, images such as bar codes, text, characters and graphics are printed onto the print media as it passes through the region.
Low performance thermal printers are relatively quiet, allowing for their use in offices, hospitals and other environments where excessive noise would be undesirable. High performance thermal printers are faster and print with at a higher print quality than low performance thermal printers. Unfortunately, this increase in speed and quality comes at the cost of a higher external noise output. The noise outputs for high performance thermal printers may reach or exceed 79 dB (approximately the noise level of busy city traffic) making high performance thermal printers undesirable for use in offices, hospitals or other environments where noise is a concern.
Prior attempts to reduce noise emission in thermal printers have been inadequate. For example, it is known that reducing the print speed reduces noise output, but this also reduces the performance of the thermal printer. Also, some noise can be reduced by changing the pressure/alignment relationship of the print head to the paper; however, this is unfavorable due to heat transfer, media flexibility, and/or cost limitations. Soundproofing materials have also been added to the printer, but relying solely on soundproofing methods increases the cost and weight of the thermal printers and is further limited by cooling limitations. A further limitation of soundproofing methods is that they only achieve maximum effectiveness at relatively high frequencies.
In other fields, noise cancellation has been achieved by fixing a speaker at a position relatively close to a listener and emitting an inverted cancellation signal towards the direction of the listener. For example, in one prior art approach, a microphone is positioned on a set of headphones to receive sound waves before they reach the ears of the listener. The sound waves are inverted and played through the speakers of the headphones to cancel out the noise. Inverted signals have also been used to cancel the engine noise in the interior of an automobile. Signals from the engine are used as inputs to a signal generator which outputs an inverted signal to a speaker on the interior of the automobile. In electronic devices, noise cancellation has been implemented to cancel noise output from the back of a cooling fan. A microphone is mounted in the air plenum of the cooling fan and a speaker is fixed relatively close to the back of the fan. The output signal from the microphone is used to drive the speaker inversely to the measured output of the fan.
The prior art approaches described above do not solve the problem of high noise emissions from a thermal printer. Each of the noise cancellation approaches described above is directed to unidirectional noise cancellation, with a speaker at a fixed position close to the listener. These approaches would be undesirable in a thermal printer. For example, it would not be practical for every person in an office to wear headphones or to physically separate the printer from potential listeners. Further, unlike the cooling fan which produces unidirectional noise (from a single noise source with flat wavefronts through a duct out the back of the device) a thermal printer emits noise in various directions from many noise sources, and can be heard by listeners from all sides of the thermal printer and at various distances from the thermal printer.
Thus it would be desirable to provide a simple and inexpensive apparatus for a thermal printer that is capable of omnidirectional noise cancellation without sacrificing printer performance.